A thin film transistor (TFT) is a kind of field effect transistor (FET). The basic structure of a TFT is a three-terminal element that includes a gate terminal, a source terminal and a drain terminal, and is an active element that has a function of using a semiconductor thin film that is formed on a substrate as a channel layer in which electrons or holes move, and when voltage is applied to the gate terminal, controls the current that flows in the channel layer and switches the current between the source terminal and drain terminal.
Currently, polycrystalline silicon film or amorphous silicon film is used as the channel layer of a TFT. Particularly, amorphous silicon film can be formed as a uniform film on a large area tenth generation glass substrate, so is widely used as a channel layer of a TFT for liquid crystal panels. However, the mobility of electron carriers (carrier mobility) is a low 1 cm2/Vsec or less, so application in TFT for high-definition panels is becoming more difficult. In other words, as the definition of liquid crystals increases, there is a need to use semiconductor thin film that has higher carrier mobility than the 1 cm2/Vsec carrier mobility of amorphous silicon film in the channel layer.
On the other hand, polycrystalline silicon film has high carrier mobility of about 100 cm2/Vsec, so has sufficient characteristics as channel layer material for a TFT for a high-definition panel. However, in polycrystalline film, the carrier mobility at the crystal boundary is reduced, so there are problems in that the uniformity in the surface of the substrate is poor, and there is variation in the TFT characteristics. Moreover, the production process for a polycrystalline film is such that after an amorphous silicon film is formed at a relatively low temperature of 300° C. or less, the film is crystallized by an annealing process. This annealing process is a special process that employs excimer laser annealing or the like, so a high running cost is necessary. In addition, the size of the glass substrate that can be used is fixed at a fifth generation substrate, so the cost reduction is limited, and thus product development is also limited.
Therefore, currently there is a need for a channel layer material that includes the best characteristics of both amorphous silicon film and polycrystalline silicon film, and that is also low cost.
In regard to this, JP 2010-182924 (A) discloses a transparent amorphous oxide thin film (a-IGZO film) that is formed by a vapor phase film formation method, and includes the elements In, Ga, Zn and O, where the composition of the oxide after crystallization is InGaO3(ZnO)m (m is a natural number less than 6), and a thin film transistor in which this transparent semi-insulating amorphous oxide thin film is taken to be the channel layer. This transparent semi-insulating amorphous oxide thin film is capable of carrier mobility greater than 1 cm2/Vsec, and a carrier density of 1016/cm3 or less without adding impurity ions.
However, even though the a-IGZO film that is disclosed in JP 2010-182924 (A) and formed by a vapor phase film formation method such as a sputtering method or pulse laser vapor deposition method has relatively high carrier mobility in the range 1 to 10 cm2/Vsec, in the amorphous oxide thin film oxide deficiencies inherently occur easily, and the behavior of the electron carriers is not always stable with respect to external factors such as heat, so there is a problem in that the operation of a device such as a TFT becomes unstable. Furthermore, it has been pointed out that when a negative bias is continuously applied to a TFT element under visible-light irradiation, a phenomenon, which is unique to amorphous film, occurs in that the threshold voltage shifts to the negative side (illumination negative bias degradation phenomenon), and this phenomenon becomes a serious problem in uses such as liquid-crystal displays.
On the other hand, JP 2008-192721 (A) or JP 2010-251606 (A) disclose applying an indium oxide film that is doped with tin, titanium or tungsten, or an indium oxide film that is doped with tungsten and zinc and/or tin for the channel layer with the object of obtaining a thin film transistor in which elements can be made on a polymeric substrate without requiring a high-temperature process, and that is capable of achieving high performance and high reliability at low cost. These technologies take advantage of the tendency to support an amorphous property and the heat stability or film flatness of indium oxide film that is doped with tungsten, or an indium oxide film that is doped with tungsten and zinc. These oxide thin films are obtained by sputtering film formation without heating the substrate, and since an annealing process is not performed after film formation, the film is amorphous film. As a result of applying these amorphous oxide thin films to the channel layer, the TFT element achieves carrier mobility of about 5 cm2/Vsec. However, these amorphous oxide thin films also, while being amorphous films, have problems in that oxygen deficiencies occur easily, and are unstable with respect to external factors such as heat, and furthermore, have a problem in that the illumination negative bias degradation phenomenon, which is unique to amorphous film, occurs.